Stereotactic radiosurgery provides a dose of radiation in a target volume in a patient. The target is irradiated at a multiplicity of orientations with finely collimated beams. The use of stereotactic radiosurgery to render tissue necrotic is well established and various systems are currently used for stereotactic radiosurgery. The prior art recognizes the need to confine radiation as much as possible to the target volume being treated. Generation of a desired dose pattern at the target volume is the objective of a treatment plan which takes into account limitations of the particular radiosurgical system used.
A typical implementation of LINAC (linear accelerator) scanning involves rotating the LINAC gantry about its horizontal axis which intersects the beam (orthogonally) and the vertical rotational axis of a couch at an isocenter. The patient is placed on the couch such that the target volume also intersects the isocenter. The gantry rotation causes a beam of radiation to trace an arc on a sphere surrounding the target. The heavy rotating gantry and couch are associated with added expense and reduced accuracy.
Another prior art implementation of LINAC beam scanning is by rotating the patient about a vertical axis which intersects the target and irradiating with a beam which is angled with respect to the axis. For example, U.S. Pat. No. 5,250,019 describes a radiotherapy system oriented toward irradiating patient's head. The system utilizes a stationary source, means for rotating the patient and means for positioning said patient's head.
There are disadvantages of the system of U.S. Pat. No. 5,250,019, such as for performing Intensity Modulated Radiotherapy (IMRT). For example, IMRT requires a precise mechanical coupling of the radiation beam to the patient's rotational axis, and target immobilization relative to the patient's rotational axis. The required precise mechanical coupling can be somewhat achieved, but with a relatively expensive subsystem. The immobilization requirement limits the application to the head only, whereby the target's position is fixed relative to a head frame via an invasive attachment, applicable only for a single use. As a result, fractionated treatments, which are required in performing IMRT, cannot be supported by such a system.
An additional shortcoming of the known radiotherapy systems is the need for a shielded vault in which the treatment takes place. Construction of such a vault is expensive and time consuming due to size requirements. Gantry and couch rotations result in radiation scattered into a large volume that has to be shielded. Large volume shielding prohibits usage of high quality shielding materials such as tungsten or lead due to the high costs.